1. Field of the Invention
The invention relates to a valve drive system that uses an electric motor for driving valves provided in cylinders of an internal combustion engine, and also relates to a method of driving the valves.
2. Description of the Related Art
As one type of valve drive systems, a valve drive system that uses an electric motor for driving (i.e., opening and closing) valves provided in cylinders of an internal combustion engine is widely known. In the valve drive system of this type, it is necessary to synchronize rotation of the crankshaft and rotation of cams with high accuracy, from the viewpoint of avoiding interference between pistons and the valves. JP-A-2005-054732 discloses a valve drive system that interrupts or cuts off power transmission to the valves so as to stop opening and closing motion of the valves when synchronism between the crankshaft and the cams is lost for some reason, namely, when rotation of the crankshaft and rotation of the cams are out of synchronism. JP-A-2005-054732 also discloses a valve drive system in which cams that provide a high valve lift and cams that provide a low valve lift are prepared, and the low-lift cams are used in place of the high-lift cams when the crankshaft and the cams are out of synchronism, so that the lift amounts of the valves can be reduced.
In the system as disclosed in JP-A-2005-054732, the low-lift cams for reducing the lift amounts of the valves must be prepared, and a mechanism for switching from the high-lift cams to the low-lift cams must be provided. Also, there is a need to provide a mechanism that interrupts transmission of power to the valves so as to stop opening and closing motion of the valves. Thus, the system of JP-A-2005-054732 may suffer from increased complexity in construction and increased cost.
In some cases, the range of movements of the valves needs to be restricted irrespective of whether the rotation of the crankshaft and the rotation of the cams are in or out of synchronism with each other. If the system of JP-A-2005-054732 is employed in such cases, the range of movements of the valves may be restricted, but the cams can freely rotate by themselves, thus allowing the electric motor to continue rotating. Therefore, if any abnormality occurs to a motor driving system that extends from the electric motor to the valves, the abnormality may become more serious as the motor keeps rotating.